It is well-established that substituted phenols such as butyl, amyl, and phenyl phenol react with formaldehyde to form modifiers for oil and alkyd varnishes. These modifiers have been used for many years. Phenolics are used to upgrade corrosion properties, improve adhesion, and improve substrate wetting. They can be "cooked" with various drying oils or simply cold blended with oils or alkyds to produce spar varnishes and metal primers. Although phenolics having some excellent performance properties, such as excellent adhesion and good corrosion properties, other properties are not so desirable. Phenolics have a relatively high viscosity which excludes their use in very low V.O.C. applications. Furthermore, phenolics turn dark in color upon aging which limits their use in some primer and most topcoat systems because of color bleed-through. For instance, the use of phenolics in exterior metal paints is limited to primers since the phenolics in such coatings darken (bleed through) with time, changing the color of light topcoats. Even when it is the primer that contains a phenolic, topcoats must be dark in color in order to not show bleed-through. Typically primers are red or gray. It is believed that darkening upon aging is caused by the formation of quinone methides in the phenolic polymer.
Phenolics having lower solution viscosities are desired in order to reduce solution viscosities in spar varnishes, bridge paints, porch and deck enamels, and government specification paints.
Phenolics are generally used in conjunction with oils or alkyds for exterior primers. Due to environmental pressures and the commercialization of new polymer systems such as urethanes, these types of phenolics represent a shrinking market.
There has been very little change in the basic chemistry of the phenolic resins since their introduction over 70 years ago. Generally, phenolic resins for oils and alkyds are novolak polymers based on substituted phenols. Originally, they were based on p-phenyl-phenol. This monomer offered a preferred combination of oil solubility, color retention and corrosion resistance. ##STR1##